This application relates to devices for measuring the properties, such as density and viscosity, of fluids and compliant solids. More particularly, it relates to the construction of such devices that are hermetically sealed against corrosive and/or conductive fluids, which could otherwise degrade the function of the sensor, and that may be used in extremes of temperature and pressure in excess of 2000 bar and temperatures up to 200° C. and higher, making them useful for process applications including downhole measurement of drilling, cementing, and formation fluids in oil, gas and geothermal exploration, completion, and production.
The method of using a vibrating elastic wire for measuring the properties of a fluid, including density and viscosity, is well known. [Vibrating Wire Viscometer; J. T. Tough, W. D. McCormick, and J. G. Dash; Rev. Sci. Instrum. 35, 1345 (1964); U.S. Pat. Nos. 8,166,812 and 7,194,902, among others]. In a typical embodiment of such a method, a conductive wire is stretched between two supports, which are electrically insulated from one another. The wire is immersed in a transverse magnetic field. A current passed through this wire results in a Lorentz force being applied to the wire, in a direction mutually perpendicular to the magnetic field, on the one hand, and to the direction of the current (in this case, the longitudinal axis of the wire) on the other.
Such a wire has a resonant frequency in air that is dependent on its density, axial tension, and to a degree dependent on the ratio of its diameter to its length, also on its elastic moduli. If an oscillating current is passed through the wire at a frequency near its resonant frequency, the wire will oscillate in a direction perpendicular to the transverse magnetic field, and will continue to oscillate even after the current is shut off. Alternatively, the wire may be excited with a step function of direct current, and will oscillate at its resonant frequency when the current is shut off.
This continuing transverse oscillation will result in a current being induced in the wire, because it is a conductor moving in a magnetic field. This induced current can be used to monitor the decay of the wire's oscillations. The decay time of the oscillations is a measure of the wire's mechanical damping, which is itself dependent on the characteristics of the wire, and more particularly, the characteristics of a fluid in which it may be immersed. The decay time of the oscillations is dependent on both the density and the viscosity of the fluid, or more specifically, on the product of density and viscosity.
In fact, any of several methods may be used to measure the damping of the wire, including but not restricted to:
1. The wire may be excited with a periodic current, and its deflection measured by other means, as for example, an optical transducer.
2. The electrical impedance of the wire may be measured over a range of frequencies near its resonant frequency, and from the complex impedance curves, together with a theoretically or empirically derived model, the viscosity and density of the fluid may be inferred.
3. The resonance of the wire may be excited by an electrical transient, and the resulting oscillation measured by the current induced in the oscillating wire.
4. The resonant wire may be made part of a gated phase-locked loop, of the kind described in U.S. Pat. No. 5,837,885 and in U.S. Pat. No. 8,291,750.
Any of these methods may be used singly or in combination with one another, the ultimate goal being to measure the damping and resonant frequency of the wire. In general, both the damping and the resonant frequency of the wire will be influenced by both the density and the viscosity of the fluid. By use of suitable empirical and/or theoretical models, the influences of density and viscosity may be separately determined, and these two properties derived from the measured damping and resonant frequency of the vibrating conductor.
This system has several disadvantages:
1. Its use is largely restricted to non-conductive fluids. Conductive fluids, such as salt solutions, will provide an alternative current path, perhaps even “short circuiting” both the driving current as well as the induced signal current.
2. In order to accurately measure the density of the fluid, the change in resonant frequency due to the fluid must be measured accurately. However, the “base” frequency—the frequency of the wire not loaded by fluid mass—must be known. This frequency is dependent on the tension of the wire. The two ends of the wire must be electrically insulated from one another. That means that the “mechanical circuit” comprising the wire and its support, will consist of materials with differing characteristics. This makes it complicated to predict the change of tension of the wire as a function of temperature.
3. The resonant frequency of the wire is determined by its density, length and axial tension. Generally, wires used for such devices must be very thin, making them vulnerable to mechanical damage, as by particles that may be present in the fluids whose characteristics are to be measured.
Some known techniques for attempting to address these problems include:                Providing an insulating coating for the wire, so as to avoid current flowing through the fluid. Such a coating may increase the damping of the wire, as well as the change in damping with temperature. Also, insulating coatings are seldom free of pores, and have a tendency to peel off with time, changing the mass of the wire and increasing its vulnerability to electrical conductivity and corrosive action of the fluid.        Making the insulating member of a material whose thermal coefficient of expansion matches that of the wire, making it simpler to predict the effect of temperature on the wire's resonant frequency. Such matching of expansion coefficients severely restricts the range of available materials, as well as typically being accurate over only a limited range of temperatures.        
Another variant of this basic system relies on the elasticity of the conductor, rather than its axial tension. This can be achieved by forming the conductor into a loop whose ends are anchored in an insulating material. The loop acts mechanically as a beam-like structure, whose resonant modes are dependent on its elastic properties and its density. Such a system has been described in U.S. Pat. No. 8,291,750.
This improvement removes the restriction of making the resonant frequency dependent on the wire's tension, but leaves the problem of a bare or insulated wire being vulnerable to the fluid in which it is immersed. Also, the resonant frequency is still dependent on the mass and elasticity of the conductor, restricting the characteristics of the system to those dictated by the properties of the conductor.
A further restriction on the use of this device is that the electrical connections to the wire loop are themselves immersed in the fluid. In addition to the above-mentioned problems that may be created by insulating the wire and its connections, these connections become especially problematical when the device is to be used in high-pressure applications, such as in downhole fluid measurements in deep-hole drilling, such as in oil, gas, and geothermal exploration and production. In such applications, the electronics package that drives and monitors the sensor must be maintained at near-atmospheric pressure in a dry environment, which necessitates passing the leads of the sensor through a pressure barrier. Such feed-through devices must make a hermetic seal between the conductor, the insulator and the pressure barrier. Such seals are typically composed of polymeric resins that have temperature-dependent elastic properties and that therefore produce undesired temperature-dependent effects on the damping and resonant frequency of the loop.